1. Field of the Invention
This invention relates to a radiation image read-out method used in a radiation image recording and reproducing system. This invention particularly relates to a radiation image read-out method for correcting final read-out conditions and/or image processing conditions adjusted on the basis of image information obtained by preliminary read-out in a radiation image recording and reproducing system wherein preliminary read-out and final read-out are carried out.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulale phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
One embodiment of the aforesaid radiation image recording and reproducing system is disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the embodiment, before final read-out for scanning the stimulable phosphor sheet carrying a radiation image of an object stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, detecting the emitted light by a photoelectric read-out means and converting it into an electric image signal is conducted, preliminary read-out for approximately detecting the image information stored in the stimulable phosphor sheet is conducted by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out. Read-out conditions for the final read-out (hereinafter referred to as the final read-out conditions) and/or image processing conditions used in image processing of the electric image signal obtained by the final read-out (hereinafter simply referred to as the image processing conditions) are adjusted on the basis of the image information obtained by the preliminary read-out (hereinafter referred to as the preliminary read-out image information).
As described above, the level of the stimulating rays used in the preliminary read-out should be lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays in the final read-out, the output of the stimulating rays source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM, or the like positioned on the optical path. Alternatively, a stimulating ray source for the preliminary read-out may be positioned independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
In image read-out by scanning a stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays, it has theretofore been considered that the amount of light emitted per unit area of the stimulable phosphor sheet is proportional to the stimulating ray energy per unit area thereof.
This conventional assumption will be explained with reference to FIGS. 1A and 1B which are schematic views showing the conditions of scanning a unit area 2 at the same position of the same stimulable phosphor sheet 1 carring the same radiation image stored therein by stimulating rays at a low read-out line density and at a high read-out line density of the stimulating rays, FIG. 2 which is a graph showing the relationship between the read-out line density and the amount of light emitted per picture element, and FIG. 3 which is a graph showing the relationship between the stimulating ray energy per unit area of the stimulable phosphor sheet and the amount of light emitted per unit area of the stimualble phosphor sheet. Specifically, it has heretofore been considered that, if the stimulating ray energy per picture element 4 is maintained the same when the density of read-out lines 3 of the stimulating rays is doubled as shown in FIGS. 1A and 1B, the amount of light emitted per picture element 4 does not change as indicated by the chain line in FIG. 2 but only the number of picture elements per unit area changes in proportion to the square of the read-out line density. Since
the stimulating ray energy per unit area=stimulating ray energy per picture element.times.number of picture elements per unit area, PA1 the amount of light emitted per unit area=amount of light emitted per picture element.times.number of picture elements per unit area,
and
the stimulating ray energy per unit area and the amount of light emitted per unit area change in proportion to the square of the read-out line density. Therefore, as indicated by the chain line in FIG. 3, if the stimulating ray energy per picture element is maintained the same when the read-out line density is changed, the amount of light emitted per unit area changes in proportion to the stimulating ray energy per unit area with the ratio of the amount of light emitted per picture element to the stimulating ray energy per picture element being the proportionality factor.
Therefore, in the radiation image recording and reproducing system wherein the preliminary read-out and the final read-out are carried out, even when the read-out line density in the final read-out (hereinafter referred to as the final read-out line density) is changed, the change in the final read-out line density has not heretofore been considered, and the final read-out conditions and/or image processing conditions have been adjusted on the basis of the preliminary read-out image information by presuming that the amount of light emitted per picture element is always the same if the stimulating ray energy per picture element is the same.
However, experiments conducted by the inventors revealed that, if the read-out line density of stimulating rays is changed when image read-out is carried out by scanning the unit area 2 present at the same position of the stimulable phosphor sheet 1 carrying the same radiation image by stimulating rays as described above, the amount of light emitted per picture element changes even when the stimulating ray energy per picture element is the same. For example, when the read-out line density is increased from that shown in FIG. 1A to that shown in FIG. 1B, the amount of light emitted by one picture element 4 in FIG. 1B becomes smaller than the amount of light emitted by one picture element 4 in FIG. 1A as indicated by the solid line in FIG. 2. Therefore, as indicated by the solid line in FIG. 3, the amount of light emitted per unit area 2 is not proportional to the stimulating ray energy per unit area 2.
Accordingly, in the radiation image recording and reproducing system wherein preliminary read-out and final read-out are carried out, even when the radiation image stored in the stimulable phosphor sheet is the same and the stimulating ray energy per picture element is the same, the amount of light emitted per picture element becomes different if the final read-out line density is different. As a result, the image information obtained by the final read-out (hereinafter referred to as the final read-out image information), i.e. the final read-out electric image signal becomes different. Therefore, when the final read-out conditions and/or image processing conditions are adjusted on the basis of the preliminary read-out image information by ignoring the change in final read-out line density, the final read-out image information and/or the image signal after the image processing is caused to change undesirably by the change in final read-out line density.
The picture element 4 is a division of a predetermined length present on the read-out line 3 of the stimulating rays. The predetermined length changes in accordance with the read-out line density and is, in general, inversely proportional to the read-out line density. Therefore, in general, the number of picture elements per unit area is proportional to the square of the read-out line density.